Water systems such as pools and spas have become increasingly popular in private homes, hotels, fitness centers, and resorts. To ensure that the water systems can be enjoyed safely, pool and spa water must be treated to reduce or eliminate chemical oxygen demands (COD) and/or total organic carbon (TOC) in the water. A common ingredient for treating water systems is potassium monopersulfate (PMPS), which is typically available in the form of a triple salt, (KHSO5)x.(KHSO4)y.(K2SO4)z (herein referred to as “PMPS triple salt”). The strong oxidation potential of PMPS triple salt makes it effective for decreasing the concentration of COD.
When treating water with PMPS, a high concentration of PMPS is added to the water to “shock” treat the water. A typical shock treatment dosage may be, for example, one pound of PMPS triple salt per 10,000 gallons of water per week. Although increasing the dosage makes the treatment more effective, the dosage cannot be increased beyond two pounds per 10,000 gallons of water per week because of the presence of potassium oxodisulfate (K2S2O8), an irritating byproduct of the PMPS triple salt. Potassium oxodisulfate, which is a harsh irritant with a long half life, is inherent in most commercially available PMPS products (e.g., Oxone®). To minimize the likelihood of bathers coming in contact with potassium oxodisulfate, the shock treatment is usually performed at least half an hour before the pool/spa is to be used.
Although this shock treatment method is highly inconvenient because of the necessary interruption of the pool/spa usage, it is a prevalent method of treatment because it minimizes bathers' contact with irritating components of the PMPS product. Potassium oxodisulfate is especially problematic not only because of its highly irritating quality but also because of its high stability. Unlike PMPS, which has a fairly short half-life at elevated pH and temperature, potassium oxodisulfate lingers around in the water long after the active ingredient of the PMPS is depleted. Potassium oxodisulfate, thus, limits the frequency of pool treatment and the method by which pools/spas can be treated. For example, pool treatment would be easier if the PMPS triple salt could be added continually, in smaller dosages, to a stream of water that circulates into the pool. However, due to the high stability of potassium oxodisulfate, applying even a small dosage of a commercially available PMPS product to the return water is likely to result in a local concentration of potassium oxodisulfate that is high enough to cause irritation.
Some physical and health consequences resulting from exposure to potassium oxodisulfate are documented in the following references:    Wrbitzky R., et al., “Early reaction type allergies and diseases of the respiratory passages in employees from persulphate production,” Int. Arch. Occup. Environ. Health, Vol. 67(6):413-7 (1995).    Le Coz, C. J., Bezard M., “Allergic contact cheilitis due to effervescent dental cleanser: combined responsibilities of the allergen persulfate and prosthesis porosity,” Contact Dermatitis Vol. 41(5):268-71 (November 1999).    “Consultation de Dermato-Allergologie,” Clinique Dermatologique des Hopitaux, Universitaires de Strasbourg 1, France.    Yawalkar, N. et al., “T cell involvement in persulfate triggered occupational contact dermatitis and asthma,” Institute of Immunology and Allergology, University of Bern, Inselspital, Switzerland.
In addition to the inconvenience of interrupted pool/spa usage, the periodic shock treatment has the problem of allowing the COD concentration to increase between shock treatments. Because the “shock treatment” cannot be performed too frequently, COD concentration can get too high for many bathers after a certain number of days from the previous treatment. During those days, water quality is compromised with increased levels of turbidity, chloroamines, and trihalomethane (THM). These byproducts of incomplete oxidation cause not only eye and skin irritation but also respiratory problems such as asthma. Moreover, these byproducts are known to cause severe corrosion of metal equipment around the pool/spa facility.
Furthermore, indirectly, potassium oxodisulfate weakens the effect of sanitizers that are used to disinfect water. Chlorine and bromine are some of the sanitizers that are commonly used for preventing viruses and bacteria from being transmitted among bathers, and chlorine is also used to oxidize any waste products produced by the bathers. In order for the antibacterial or viricidal effect to be significant, the oxidation potential of the water must be sustained above a certain threshold level. The following studies have confirmed that the effectiveness of these sanitizers is significantly reduced when contaminants is high:    S. Carlson, Fundamentals of Water Disinfection, D-8500 Nurnberg 30, Germany    K. Victorin, K. G. Hellstrom, and R. Rylander, “Redox potential measurements for determining the disinfecting power of chlorinated water,” Department of Environmental Hydiene, The National Institute of Public Health and the Institute of Hygiene, Karolinska Institute, Stockholm, Sweden (October 1971).    Frank Scully, Jr. and Angela Crabb Hartman, “Disinfection Interference in Wastewater by Natural Organic Nitrogen Compounds,” Environmental Science and Technology, vol. 30. No. 5, Department of Chemistry and Biochemistry, Old Dominion University, Norfolk Va. (1996) American Chemical Society.Although PMPS has the ability to raise the oxidation potential of the water when many contamination sources (e.g., many bathers) lower the oxidation level, PMPS cannot be used because its use might increase the oxodisulfate level in the water to a range above the recommended level. The presence of contaminants impairs the ability of the sanitizer/oxidizer to effectively sanitize the water. Also, because of competing reactions, the ability of the halogen-based sanitizer/oxidizer to rid the water of inorganic nitrogen such as mono & dichloro amines is significantly impaired.
The currently-used periodic shock feeding method does not provide for sustained disinfection rates where contaminants are added between treatments. During the interval period between shock treatments, accumulating contaminants imposes a burden on the sanitizer/oxidizer and impairs the disinfection rate due to competing reactions. Also, as already noted, the competing reactions between accumulated organics and nitrogen contaminants for the sanitizer/oxidizer allows for increased levels of chloramines which impairs both water and air quality.
To address these issues, sophisticated control and application technologies have been employed to allow for more frequent feed of PMPS while bathers are present. The following references disclose some exemplary technologies:    U.S. Pat. No. 6,620,315 and U.S. Pat. No. 6,623,647 describe a method and apparatus that combined measuring ORP and Free Available Chlorine (FAC) to independently adjust the feed of multiple oxidizers such as chlorine and PMPS.    U.S. Pat. No. 6,409,926 and U.S. Pat. No. 6,432,234 describes a means of reducing the ORP set-point used to control the feed of the halogen based sanitizer to achieve breakpoint chlorination by feeding a coagulant to reduce the contaminants on chlorine.    U.S. Pat. No. 6,143,184 describes a process for achieving continuous breakpoint halogenation by optimizing the control of halogen-based sanitizer/oxidizer using ORP control.    U.S. Pat. No. 6,149,819 describes a process for achieving continuous breakpoint halogenation using halogen donor and PMPS controlled by an ORP controller.
In order to address the drop of oxidation potential between shock treatments, ORP control technology may be used to optimize the feed of chlorine and PMPS or coagulant to reduce the contaminants, thereby reducing the competing reactions and enhancing the chlorine's ability to achieve breakpoint chlorination. The ORP control technologies, however, have their disadvantages. For example, they require expensive chemical feed and control technology as well as extensive on-site maintenance and expertise to tune in or optimize the sequencing of the chemicals being fed.
A method of cleaning water without the expense of the ORP control technologies and restrictions of the shock treatment is desired.